CNC machining requires the control of a number of variables in order to attain maximum productivity for a given cutting operation. The severity of the cutting operation being performed will change the rate of feed or depth of cut. For example, broaching is a more rigorous cutting operation than sawing.
Metals have various machining ratings. The hardness influences a workpiece's machinability. Steel manufactured bars will be either hot rolled or cold drawn, which affects the hardness. A hardness in excess of 400 Brinell is not uncommon. For example, cold drawn plain carbon steels, containing less than three tenths of one per cent (0.3%) carbon, have better machinability than a hot rolled steel in a carbon range of between 0.3% to 0.4%. But there is little difference in the machinability between cold rolled or hot rolled steels until the carbon content becomes more than 0.4%, and then the hot rolled steel has a machinability superior to that of a cold rolled or cold drawn steel having 0.4% carbon or more. The alloy composition, and physical properties due to heat treating, tempering or annealing changes the hardness and thus machinability. Alloy steels such as stainless steel have lower machinability ratings and will require slower cutting speeds, shallower depths of cut, higher cooling rates, and more efficient coolants than plain carbon steel to avoid the buildup of stress in the part and overheating the cutting tool.
Uniform chips that can be carried away by a flood of coolant to maintain the work area relatively clean of chip debris is important. More important still is the shape and color of the chips. Chips that exhibit breaking in discontinuous curls of a "6" or "9" shape when removed are indicative of good temperature control. Blue or blackened chips are a sign of excessive temperature and stress buildup in the tool and workpiece, shortening tool life and introducing surface irregularities to the workpiece.
Temperature buildup in the tool can quickly destroy the cutting edge geometry of an expensive tool insert where cooling is critically important for tool life. The speed of cutting will usually provide a fairly good surface quality to the workpiece minimizing the need for lubrication. As a general guideline, an insert will have a twenty minute cutting life under normal conditions before it must be flipped over to provide a new cutting face or edge geometry, before being replaced entirely with a new insert depending upon the finish tolerances allowed for the cutting operation. An insert will be selected according to the metal's machinability, type of finish required, feed rate, cutting depth and rake angle. An Insert is fastened in a tool bar which has a socket formed in one end for holding the insert rigidly at the desired rake angle, relative to the rotating workpiece. The tool bar is advanced by the turret head at a prescribed feed rate, and depth of cut, which are dimensionally pre-programmed factors in the setup of the machine tool. The more rigorous the set up conditions, the more coolant volume and pressure that are required to prevent excessive tool wear.
Since cooling tends to be a controllable factor essentially independent of the other setup conditions, reducing tool wear is directly proportional to the volume and pressure of the coolant. CNC machines have a coolant pump and reservoir of given capacity. A coolant system capacity of about eight gallons per minute is common. A delivery pressure of thirty (30) psi is typical, however auxiliary systems are available which increase the delivery pressure. But even with the higher pressures and cooling capacities of auxiliary systems, tool life remains the chief limiting factor in the productivity and cost reduction of most machining operations.
As a result, many CNC machine operators, in order to remain competitive, have attempted to get more efficiency out of their existing machine tool's cooling system by directing more of the coolant onto the tool insert by coolant bored holders and bars.
Coolant pumped to the turret is distributed to each tool station where a coolant bored tool holder may be used that has internal coolant passages that can direct more coolant toward the work than the conventional tool holder.
Also, center bored tool bars may add coolant more precisely to the insert. A port at the tool end near the insert may augment coolant delivery to tile tip, but longitudinal gaps still exist between the cylindrical bore of the tool holder and outer surfaces or diameter of the tool bar which may have a non-circular cross section creating a discontinuity, or one or more longitudinal gaps, between the tool bar and a clamping collar in the bore of the tool holder for securing the tool bar preventing any pressure being applied other than the existing output pump pressure.
Unfortunately, the amount of coolant that floods through the gaps, or bar bore to actually reach the cutting tool is problematical considering that the pressure is limited by the openness of the coolant delivery system. Coolant bored tool bar charts specify the the feed rate and rake angle assuming conventional coolant capacities. The delivery pressure from the center bore of the tool bar may be slightly raised by closing off the longitudinal gaps. By this technique, more coolant is forced down the center of the tool bar and projected by the port at the end directly onto the cutting edge, but still not much efficiency is gained because of the flow around and through the gaps on either side of the bar preventing the pressure from being increased beyond the existing pump pressure.
A tool bar with a coolant bore of this type is illustrated in U.S. Pat. No. 5,098,233 issued Mar. 24, 1992 to Circle Machine Company of Monrovia, Calif. U.S.A.
A coolant bored bar that is too long to be closed off inside of the tool holder must be cut off. This is a task any machine operator will try to avoid. Sawing to the proper length is necessary either to limit the tool tip projection length required since the front unsupported length should not be more than three times the diameter of the tool bar for the required tip rigidity, or the coolant bored bar will not fit behind the holder on the turret. Bars bore for cooling are expensive and cutting off the end limits the range of choices for using that bar in other setups. Choosing the right bar for the job requires balancing the set up requirements with the feed rate, depth of cut, edge geometry and other factors.
Resorting to more costly alternatives involves positioning coolant hoses at the tool tip, but piping routed around the tool holder gets in the way of the turret. Attempts to put temporary piping in place have uniformly failed. Vibration and the resultant pressures cause fittings to fail within a short time.
Consequently, accessory manufacturers have sprung up offering a separate cooling delivery systems. One such manufacturer is Slabe Machine, which markets the Max Cool.TM. attachment. This attachment includes a self contained housing separate from the tool holder. Coolant is delivered from the turret to a nozzle aimed at the tool insert at very high pressure. The nozzle focuses coolant directly on the tool tip at the exact trajectory that is optimal for both chip breaking and cooling efficiency. The coolant pressure is dramatically increased. The attachment increases it up to 5,000 psi due to the nozzle restriction. At the tool tip, where the boundary temperature gradient is greatest, the cooling efficiency is thus substantially increased. This product is an expensive solution to a long standing problem. But it is evidence of the machine industries need for higher productivity to remain competitive that such expensive alternatives are finding a market. Still the greater expense of these separate cooling systems is offset by the gains in productivity, tool life and quality. These attachments in conjunction with auxiliary pressure coolant systems increase productivity to levels unattainable with the prevailing pump pressures. Slabe Machine's web site may be found on the Internet at www..slabemachine.com./Slabe Machine Products, 4659 Hamann Parkway, Willoughby, Ohio, 44094, where a technical description is found for this attachment, for those who may be interested in a more detailed description.